MFC r204648:

MFC r204648:
Several changes to fix livelock under high load, introduced by r203489:
 - change the way in which command queue overflow is handled;
 - do not expose to CAM two command slots, used for driver's internal purposes;
 - allow driver to use up to 1024 command slots, instead of 256 before.

git-svn-id: svn://svn.freebsd.org/base/stable/8@204873 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f